Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the most recent minor version is supported.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.3.x   | :white_check_mark: |
+| < 0.3.x | :x:                |
+
+
+## Reporting a Vulnerability
+
+Please use the [GitHub private reporting functionality](https://github.com/pest-parser/ast/security/advisories/new)
+to submit potential security bug reports. If the bug report is reproduced and valid, we'll then:
+
+- Prepare a fix and regression tests.
+- Make a patch release for the most recent release.
+- Submit an advisory to [rustsec/advisory-db](https://github.com/RustSec/advisory-db).
+- Refer to the advisory in the release notes.